Biomedical subjects

A K Ulrich

Publications and source records attributed to A K Ulrich.

4 recordsLinked to original sources

PubMed1991

Codon usage, transfer RNA availability and mistranslation in amino acid starved bacteria.

The fidelity of codon reading was examined in amino acid starved Escherichia coli. In one case the level of misincorporation of methionine was measured at an isoleucine residue encoded by either the commonly used AUU codon or the rarely used AUA codon. In this situation we found the frequency of methionine misincorporation to be very low and to be unaffected by the identity of the isoleucine codon. In other experiments histidine misincorporation for glutamine was measured in glutamine starved cells with normal levels of histidine-specific tRNA and cells overproducing this tRNA. Cells overproducing the tRNA had higher levels of misincorporation.

Amino Acid Sequence↗

PubMed1989

Context specific misreading of phenylalanine codons.

It has previously been shown that the phenylalanine codon UUC encoding residue 8 of the Escherichia coli argI gene product, ornithine transcarbamylase, is misread as leucine at a high frequency during phenylalanine starvation. However, no misreading of the UUU encoding residue 3 was observed under these conditions. Using oligonucleotide-directed, site-specific mutagenesis, we have constructed mutants where these codons have been changed. Using these mutant argI genes we see a high level of mistranslation at position 8 during phenylalanine starvation whether the codon is UUU or UUC. With either codon at position 3 we see no leucine substitution. We also constructed a gene with a leucine codon at position 3. The product of this latter mutated gene is stable and active, indicating that preferential turnover of mistranslated protein is not obscuring an otherwise high rate of misreading. This would seem to indicate that it is the context rather than the particular phenylalanine codon which is important in determining these misreading levels.

Amino Acid Sequence↗

PubMed1986

Strains overproducing tRNA for histidine.

Hybridization analysis of total genomic DNA indicated that Escherichia coli K12 contains a single copy of the gene encoding the histidine-accepting tRNA. This gene was subcloned onto an inducible expression vector under the control of the tac promoter. Strains carrying the resulting plasmid showed five- to six-fold increased histidine-accepting activity after induction. This overproduction of tRNAHis did not effect the growth rate of the strain or lead to derepression of the histidine biosynthetic enzymes. Neither did it have an effect on mistranslation elicited by histidine starvation. However, in cells starved for histidine by the addition of alpha-methyl histidine, the overproduction of tRNAHis interfered with the ability of the cells to recover from starvation.

Base Sequence↗

PubMed1983

Genetic and biochemical analyses of Escherichia coli mutants altered in the temperature-dependent regulation of membrane lipid composition.

We have previously studied two mutants of Escherichia coli altered in the regulation of membrane lipid composition by temperature. One class (represented by the fabFl allele) fails to regulate upon temperature shift and is defective in cis-vaccenic acid synthesis owing to the lack of the fatty acid elongation enzyme beta-ketoacyl-acyl carrier protein synthase II(EC 2.3.1.41). A second class of mutant, given the phenotypic designation Vtr, overproduces cis-vaccenic acid at all temperatures and hence is altered in temperature regulation. In this paper we report evidence for the following conclusions. (i) The Vtr and fabFl mutations show very tight genetic linkage. (ii) The Vtr lesion is allelic to the fabFl mutation since the presence of the fabFl mutation in merodiploid strains carrying the Vtr or fabF(+) alleles results in fatty acid compositions intermediate between those of the two monoploid strains. Merodiploids carrying both the fabF(+) and Vtr alleles likewise show an intermediate composition. These results indicate intra-allelic complementation. (iii) The two E. coli proteins recently discovered by Rock (J. Bacteriol. 152:1298-1300, 1982) that form mixed disulfide cross-links to acyl carrier protein are directly demonstrated to be beta-ketoacyl-acyl carrier protein synthases I and II. (iv) The fabFl strains produce a synthase II band of altered electrophoretic mobility, indicating that the fabF locus is the structural gene for synthase II. (v) The synthase II of Vtr strains is abnormally sensitive to cerulenin, an antibiotic that specifically inhibits synthases I and II. This increased sensitivity is readily demonstrated in vivo, but in vitro we failed to detect an increased sensitivity of the Vtr synthase II to cerulenin, nor have we detected any other kinetic or structural alteration in the enzyme. We interpret these results in terms of specific interactions of synthase II with other cellular components which occur in vivo but are not duplicated in vitro.

3-Oxoacyl-(Acyl-Carrier-Protein) Synthase↗